1. Field of the Invention
The present invention relates to an automatic blood-pressure measuring apparatus for automatically measuring a blood pressure of a living subject according to so-called oscillometric method.
2. Related Art Statement
In an oscillometric blood-pressure measurement, a pressing pressure of a cuff wound around, e.g., an upper arm of a living subject, that is, a cuff pressure is slowly changed, and a blood pressure of the subject is measured based on change of amplitudes of a plurality of heartbeat-synchronous pulses of a cuff pulse wave as an oscillatory component produced in the cuff during that process, with respect to change of the cuff pressure. FIG. 1B shows the heartbeat-synchronous pulses of the cuff pulse wave extracted from the cuff pressure, and a systolic and a diastolic blood pressure values are determined based on greatest-slope points or inflection points of an envelope, indicated at one-dot chain line in FIG. 1B, that passes through the greatest one of respective amplitudes of the successive pulses of the cuff pulse wave. An automatic blood-pressure measuring apparatus that employs the oscillometric method can measure blood pressure at a region, such as anterior region of neck, finger, or inferior limb, where it is considerably difficult to measure blood pressure according to so-called microphone method. In addition, though the microphone method essentially requires that a microphone be accurately placed right above an artery, the oscillometric method just requires that a cuff be wound, and thus can be easily carried out without needing a special skill of a measuring person. Moreover, the oscillometric method can be used at noisy places, and can also be used to measure blood pressure from children who produce small Korotokoff sounds only, or patients falling in shock. Thus, the automatic blood-pressure measuring apparatus employing the oscillometric method is widely used.
As explained above, in the oscillometric method, a blood pressure is measured based on change of respective amplitudes of respective pulses of a cuff pulse wave, produced in a cuff during a slow changing of a cuff pressure, with respect to the change of the cuff pressure. However, in a particular case where arrhythmia such as atrial fibrillation occurs, as shown in FIG. 1C, a waveform of a cuff pulse wave is deformed, and accordingly an envelope connecting respective upper ends of respective amplitudes of the cuff pulse wave along the axis of cuff pressure is also deformed. Thus, a blood pressure cannot be measured, or an inaccurate blood pressure is measured.
It is therefore an object of the present invention to provide an automatic blood-pressure measuring apparatus which can measure blood pressure with high accuracy even in the case where arrhythmia occurs.
The inventors have made extensive studies to achieve the above-indicated object, and have found the following facts: In the oscillometric blood-pressure measuring method, a cuff pressure, i.e., a pressure in a cuff to press, e.g., an upper arm of a living subject is slowly changed (e.g., lowered), as shown in FIG. 1A, then a pulsatile component mixed with the cuff pressure is extracted as a cuff pulse wave, and a systolic and a mean blood pressure value are determined based on the cuff pressure and an envelope of the cuff pulse wave, i.e., oscillometric waveform, as shown in FIG. 1B. The change of amplitudes in the oscillometric waveform can be explained using an S-shaped non-linear relationship between inner volume and inner pressure of blood vessel, shown in FIGS. 2A and 2B. More specifically described, as shown in FIG. 2A, when pressure difference between inner and outer pressures of blood vessel is around zero, inner volume of the blood vessel largely changes; and, as shown in FIG. 2B, as the inner pressure increases or decreases, the amount of deformation of the blood vessel""s wall decreases and accordingly the change of inner volume decreases. Therefore, when the cuff pressure is around a mean blood pressure of the subject, the cuff (i.e., volumetric) pulse wave shows the greatest amplitude corresponding to the greatest pulse pressure (=systolic blood pressurexe2x88x92diastolic blood pressure). As the cuff pressure is changed in a direction away from the mean blood pressure, respective amplitudes of respective pulses of the cuff pulse wave become smaller. Thus, the relationship shown in FIGS. 2A and 2B can explain that when arrhythmia occurs and accordingly an intravascular pressure pulse wave changes and/or a pulse pressure shifts, a cuff pulse wave, i.e., an oscillometric pulse wave that periodically changes with inner volume of blood vessel is deformed and therefore an envelope defined by the cuff pulse wave is also deformed. This envelope cannot be used as a basis for obtaining an accurate blood pressure. Hence, respective amplitudes of successive pulses of a finger-tip pulse wave as a volumetric pulse wave that is detected by a photoelectric-pulse-wave sensor, are used in place of the respective pulse pressures of the successive pulses of the intravascular pressure pulse wave; respective correction factors are determined, according to the relationship shown in FIGS. 2A and 2B, based on respective changes of the respective amplitudes of the successive pulses of the finger-tip pulse wave from the amplitude of the specific pulse at the time of detection of the cuff pressure (i.e., the mean blood pressure) corresponding to the greatest amplitude of the cuff pulse wave; those correction factors are used to correct the respective amplitudes of the corresponding pulses of the cuff pulse wave; and a blood pressure is determined based on an envelope defined by the thus corrected amplitudes of the cuff pulse wave. This blood pressure measurement can be carried out even when arrhythmia occurs, and the accuracy of measured blood pressure value is greatly improved. The present invention has been developed based on these facts.
The above object has been achieved by a first aspect of the present invention according to which there is provided an oscillometric automatic blood-pressure measuring apparatus, comprising an inflatable cuff which is adapted to be wound around a first portion of a living subject, so as to detect respective amplitudes of a plurality of heartbeat-synchronous pulses of a cuff pulse wave as an oscillatory component that is produced in the cuff in synchronism with heartbeats of the subject in a cuff-pressure change process in which a cuff pressure as a pressure in the cuff is changed; a cuff-pulse-wave-amplitude correcting means for correcting the respective amplitudes of the respective pulses of the cuff pulse wave, according to a predetermined non-linear relationship between difference between cuff pressure and subject""s mean blood pressure, and amplitude of cuff pulse wave; and a blood-pressure determining means for determining a blood pressure of the subject based on a change of the corrected amplitudes of the cuff pulse wave.
According to this aspect, the cuff-pulse-wave-amplitude correcting means corrects the amplitudes of the cuff pulse wave, according to the non-linear relationship between difference between cuff pressure and subject""s mean blood pressure, and amplitude of cuff pulse wave, and the blood-pressure determining means determines the blood pressure of the subject based on the change of the corrected amplitudes of the cuff pulse wave. Thus, even in the case where arrhythmia occurs, a reliable envelope is obtained and accordingly a blood pressure can be measured or an accurate blood pressure can be obtained.
According to a second aspect of the present invention, there is provided an oscillometric automatic blood-pressure measuring apparatus, comprising an inflatable cuff which is adapted to be wound around a first portion of a living subject, so as to detect respective amplitudes of a plurality of heartbeat-synchronous pulses of a cuff pulse wave as an oscillatory component that is produced in the cuff in synchronism with heartbeats of the subject in a cuff-pressure change process in which a cuff pressure as a pressure in the cuff is changed; a pulse-wave detecting device which is worn on a second portion of the subject and which detects, from the second portion, a plurality of heartbeat-synchronous pulses of a pulse wave; a pulse-pressure-correction-factor determining means for determining, based on respective amplitudes of the respective pulses of the pulse wave detected by the pulse-wave detecting device, respective pulse-pressure correction factors to correct respective pulse pressures corresponding to the respective amplitudes of the respective pulses of the pulse wave, to a single value; a mean-blood-pressure storing device which stores a mean blood pressure of the subject; a relationship determining means for determining, based on the mean blood pressure stored by the mean-blood- pressure storing device, a non-linear relationship between difference between cuff pressure and subject""s mean blood pressure, and amplitude of cuff pulse wave; a mean-blood-pressure estimating means for estimating, based on the respective pulses of the pulse wave detected by the pulse-wave detecting device, respective estimated mean blood pressure values corresponding to the respective pulses of the cuff pulse wave; a mean-blood-pressure-correction-factor determining means for determining, according to the non-linear relationship determined by the relationship determining means, respective mean-blood-pressure correction factors to correct respective amplitude changes corresponding to respective deviations of the respective estimated mean blood pressure values corresponding to the respective pulses of the cuff pulse wave, from the mean blood pressure stored by the mean-blood-pressure storing device; a cuff-pulse-wave-amplitude correcting means for correcting the respective amplitudes of the cuff pulse wave, based on the respective pulse-pressure correction factors determined by the pulse-pressure-correction-factor determining means and the respective mean-blood-pressure correction factors determined by the mean-blood-pressure-correction-factor determining means; and a blood-pressure determining means for determining a blood pressure of the subject based on a change of the corrected amplitudes of the cuff pulse wave.
According to the second aspect, the pulse-pressure-correction-factor determining means determines, based on amplitudes of the pulse wave, respective pulse-pressure correction factors to correct pulse pressures corresponding to the amplitudes of the pulse wave, to a single value, the relationship determining means determines, based on the mean blood pressure, a non-linear relationship between difference between cuff pressure and subject""s mean blood pressure, and amplitude of cuff pulse wave, the mean-blood-pressure estimating means estimates, based on the pulse wave, respective estimated mean blood-pressure values corresponding to respective pulses of the cuff pulse wave, the mean-blood-pressure-correction-factor determining means determines, according to the relationship, respective mean-blood-pressure correction factors to correct respective amplitude changes caused by respective deviations of the respective estimated mean blood pressure values corresponding to the respective pulses of the cuff pulse wave, from the mean blood pressure, the cuff-pulse-wave-amplitude correcting means corrects the amplitudes of the cuff pulse wave, based on the respective pulse-pressure correction factors and the respective mean-blood-pressure correction factors, and the blood-pressure determining means determines the blood pressure of the subject based on the change of the corrected amplitudes of the cuff pulse wave. The envelope defined by the corrected cuff pulse wave is accurate, and the blood pressure can be easily determined based on the accurate envelope. Thus, even in the case where arrhythmia occurs, a blood pressure can be measured or an accurate blood pressure can be obtained.
Preferably, the present apparatus further includes a temporary-mean-blood-pressure determining means for determining, as a temporary mean blood pressure of the subject, a cuff pressure corresponding to the greatest one of the amplitudes of the cuff pulse wave by using an envelope defined by the amplitudes before correction, and the mean-blood-pressure storing device stores, as the mean blood pressure of the subject, the temporary mean blood pressure determined by the temporary-mean-blood-pressure determining means. According to this feature, an accurate mean blood pressure of the subject is obtained and accordingly a blood pressure can be measured with high accuracy even in the case where arrhythmia occurs.